HOW to handle a situation where 3 tables have on to many relationship with one single table?

Question

Hi fellows!
i have three tables named
Teacher_T(UserName, Pwd, Phone, Email, Experience, Academics) //here UserName is the PK
Student_T(UserName, Pwd, Phone, Email) //here UserName is the PK
Ad_T(UserName, Title, Description) //here UserName is the FK
i want to enable both the teacher and the student to post their ads and in the Ad_T table ... for which i have created 2 relationships in Ad_T table:
1) among Ad_T and Teacher_T using the UserName of Teacher_T as FK in Ad_T
2) among Ad_T and Student_T using the UserName of Student_T as FK in Ad_T
but this would not work as FK constraint requires that the value must be corresponding to a row in the PK table and problem here is that i am having two PK tables instead of one ... 
one way to solve this problem is to create two separate tables         1) Student_Ad_T           2)Teacher_Ad_T         but i dont want to do so .......
Any suggestions? 
farooq.hnf

Answer 1

Try:
CREATE TABLE Teacher_T(UserName varchar(30) PRIMARY KEY, Pwd varchar(30),
Phone varchar(30), Email varchar(30), Experience varchar(30), Academics varchar(30));
CREATE TABLE Student_T(UserName varchar(30) PRIMARY KEY, Pwd varchar(30), Phone varchar(30), Email varchar(30))
CREATE TABLE Ad_T(UserNameT varchar(30) NULL REFERENCES Teacher_T,
UserNameS varchar(30) NULL REFERENCES Student_T, Title varchar(30), Description varchar(30))
Kalman Toth Database & OLAP Architect
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